Edge formation process for aluminum solid electrolytic capacitor

ABSTRACT

A solid electrolytic capacitor comprising a foil coated with a dielectric oxide film, wherein the coated foil has slit or cut edges, and the slit or cut edges have been reformed by forming the foil in an aqueous citrate electrolyte, then depolarizing the foil, and then forming the foil in an aqueous phosphate electrolyte wherein the foil is not anodized in an aqueous acid electrolyte prior to forming the foil in an aqueous citrate electrolyte.

This application is a divisional application of U.S. Ser. No.09/874,388, filed Jun. 6, 2001 now U.S. Pat. No. 6,548,324, which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to an edge formation process for aluminum solidelectrolytic capacitors.

BACKGROUND OF THE INVENTION

Electrolytic capacitors with excellent high frequency characteristicsare in high demand due to speed requirements of circuits for devicessuch as computers and wireless communications. In addition, highcapacitance is required in the low voltage circuits that are used inthese devices. Conductive polymers such as polypyrrole, polyaniline,polythiophene, and their derivatives, are finding increasing use ascathodes for electrolytic capacitors because such polymers have muchhigher conductivity than the liquid electrolytes and manganese dioxidecathodes currently used in these capacitors.

A wet electrolytic capacitor has an anode metal, a dielectric, a liquidelectrolyte, and a cathode. Valve metals such as tantalum, aluminum, andniobium are particularly suited for the manufacture of high surface areaelectrolytic capacitors. The valve metal serves as the anode, and anoxide of the valve metal, coated by electrochemical oxidation of thevalve metal surfaces, serves as the dielectric. The process ofelectrochemically coating a valve metal with a dielectric oxide iscalled formation. In order to maximize the dielectric surface area, andhence increase the volumetric efficiency of the capacitor, the valvemetal substrates are porous bodies. These porous bodies can take theform of etched foils or slugs of compressed powder. The liquidelectrolyte is impregnated into the porous body. A high surface areacathode completes the circuit. Etched aluminum foil is a particularlypreferred anode material for wet electrolytic capacitors.

In the manufacture of wet aluminum electrolytic capacitors, the aluminumfoil is etched to high surface area, coated with a dielectric oxidefilm, slit to the proper width, and then cut to length. During theslitting and cutting-to-length operations, the dielectric oxide on theedges of the foil is damaged and bare aluminum is exposed. The foil isthen wound, placed in a can (along with the cathode), and filled with anon-aqueous fill electrolyte. The non-aqueous fill electrolyte iscomposed of, for example, borates in non-aqueous solvents containing avery small amount of water. After filling with electrolyte, the cans aresealed to prevent electrolyte from escaping and to keep additional waterout.

A critical part of conditioning a wet aluminum electrolytic capacitor isrepairing the damage to the dielectric oxide on the edges of the slitand cut-to-length foil and any damage to the dielectric oxide on theface of the foil that incurred during the winding operation. If theseedges are not re-formed, the capacitor will have a high leakage current.The non-aqueous fill electrolytes, containing a very small amount ofwater, are very efficient in re-forming oxide on the edges.

In the manufacture of a solid aluminum electrolytic capacitor with aconductive polymer cathode, the foil etching, forming, and slitting aredone in a similar manner to that of wet aluminum electrolytic capacitor.However, the conductive polymer is not efficient at re-forming adielectric film on the slit and cut edges and at repairing damaged oxideon the face. Therefore, this must be done in a separate step before theconductive polymer is impregnated into the aluminum/aluminum oxideanode.

Re-forming the slit and cut edges can be accomplished by immersing theelements in a formation bath or a series of formation baths. Therequirements for these edge formation baths are threefold: 1) They mustform a high quality dielectric oxide on the cut edges, 2) They mustrepair any damage to the dielectric oxide on the face of the clementthat was damaged during the slitting and cutting to length operation,and 3) They must not damage the dielectric oxide already on the face ofthe element. In addition, the formed dielectric oxide needs to haveexcellent hydration resistance.

Hydration resistance is critical for aluminum solid electrolyticcapacitors with conductive polymer cathodes. After impregnation with theconductive polymer, the capacitors are washed extensively in water toremove excess reactants and reactant byproducts. This washing is atelevated temperature (>50° C.). The aluminum oxide film is exposed toconditions very conducive to hydration during this washing process, and,therefore, the aluminum oxide film must have a high degree of hydrationresistance. Hydration of the oxide during the washing process, or onsubsequent storage after washing, can result in hydrated oxide in theweld zone and this hydrated oxide is difficult or impossible to weldthrough to make a good attachment to the lead frame.

A high degree of hydration resistance is also required during storage oruse of capacitors in high humidity environments. If the oxide becomeshydrated during use, the capacitor leakage current will increase, or thecapacitor can become a short circuit.

It was discovered that prior art electrolytes have deficiencies whenused for edge formation of aluminum anodes intended for use in solidaluminum electrolytic capacitors with conductive polymer cathodes. Thefill electrolytes used in wet aluminum capacitors are not suitable foruse outside a sealed can because of their toxic nature and theirpropensity to adsorb water from the air. Thus they cannot be used inopen, mass production electrolyte baths.

Electrolytes used for the production of the original aluminum oxide filmare also not completely suitable because they are designed to form oxideon a freshly etched surface or a hydrated oxide surface and not designedto form oxide on cut edges and to repair oxide on the face (cf. U.S.Pat. Nos. 3,796,644; 4,113,579; 4,159,927; 4,481,084; 4,537,665;4,715,936). In addition, compromises must be made in the selection of anelectrolyte because of the high current efficiency needed toeconomically produce a dielectric oxide over the entire etched aluminumsurface.

Slitting and cutting the foil to length mechanically damages the edgesand this mechanical damage should be repaired before or during theformation of the dielectric oxide film on the edge.

Several electrolyte systems have been considered for the edge formationof aluminum electrolytic capacitors with a solid conductive polymercathode. Low leakage current and high capacitance can be achieved byproducing a thick, porous layer on the edge using aqueous solutions ofoxalic acid, followed by forming a barrier layer with aqueous solutionsof ammonium adipate (EP 1,028,441 A1). A flowchart of this prior artedge formation process is shown in FIG. 2. The parts are first anodizedin oxalic acid, rinsed, and dried. This produces a thick, porous layeron the edge. Since oxalic acid has a low pH, it also tends to remove thevery outer layers of oxide from the surface. The parts are then formedin ammonium adipate, rinsed, and dried. This step produces a dielectricoxide on the edge. This is followed by a depolarization step and anotherformation in ammonium adipate, rinse, and dry. The resulting films areunstable toward hydration. The hydration resistance of the preexistingdielectric oxide is impaired because of the attack by oxalic acid.Neither ammonium adipate alone or the oxalic acid-ammonium adipatesystem are capable of forming a hydration resistant oxide on the edges.This leads to problems with leakage current instability in production,welding of the capacitors to the lead frame, and long-term stabilitytowards hydration. It is desirable to have an edge formation electrolytesystem, which provides a product with a hydration resistant oxide.

BRIEF SUMMARY OF THE INVENTION

It was discovered that edge formation in an aqueous citrate solutionfollowed by formation in an aqueous phosphate solution imparts highhydration resistance to the foil and results in a minimal loss ofcapacitance.

The invention is directed to a process for edge forming a slit andcut-to-length foil having a dielectric oxide film on at least onesurface comprising forming the foil in an aqueous citrate electrolyte,preferably an aqueous ammonium citrate electrolyte, depolarizing thefoil, and forming the foil in an aqueous phosphate electrolyte,preferably an ammonium dihydrogen phosphate electrolyte. Using thisformation process, a foil with excellent hydration resistance andcapacitance is produced.

The invention is further directed to a process for edge forming a slitand cut-to-length aluminum foil having a dielectric oxide film on atleast one surface comprising forming the foil in an aqueous ammoniumcitrate electrolyte, then depolarizing the foil, and then forming thefoil in an aqueous ammonium dihydrogen phosphate electrolyte wherein thefoil is not anodized in an aqueous acid electrolyte prior to forming thefoil in an aqueous ammonium citrate electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart of the process of edge formation according tothe invention.

FIG. 2 shows a prior art edge formation process.

DETAILED DESCRIPTION OF THE INVENTION

Aluminum is etched to a high surface area and formed with a dielectricoxide and then slit to a width suitable for the production of solidelectrolytic capacitors. The foil is then cut to length and welded to acarrier bar. A masking material is applied to the foil to define thearea that will be subsequently edge formed.

A flowchart of the edge formation process is shown in FIG. 1. The foilsare heat treated in an oven at elevated temperature to reduce the amountof surface hydration and to bring the foil surface to a well-definedstate of wettablity. The elevated temperature is generally from about250° C. to about 550° C. and the foils are heat treated from about 2minutes to about 12 hours. Preferably the foils are heat treated at atemperature from about 300 to about 350° C. for about 15 to 30 minutes.

The foils are first edge formed in an aqueous citrate electrolyte(1^(st) edge formation). The citrates can be soluble citrates salts ofalkali metal, amine, or ammonium cations. Preferably, the electrolyte isammonium citrate with a pH in the range of about 4 to about 9,preferably in the range of about 5 to about 7. The concentration of thecitrate in water is from about 0.1 wt % to about 10 wt %, preferablyabout 0.5 wt % to about 5 wt %, more preferably about 1 wt %. Thetemperature of the electrolyte is from about 0° C. to about 90° C.,preferably from about 50° C. to about 90° C., more preferably about 55°C. The time of formation depends on the concentration and temperatureand is typically from about 3 minutes to about 20 minutes, preferably,about 10 minutes.

The foils are then rinsed of the aqueous citrate, dried to remove excesswater, and depolarized. The depolarization step exposes any hydrate,trapped gas, or voids in the oxide produced during previous formationsteps. The foils may be depolarized by heating the foils to an elevatedtemperature or by soaking on open circuit in a hot borate or citratesolution. Preferably, the foils are depolarized by heating the foils toabout 250° C. to about 550° C., for about 30 seconds to about 2 hours,preferably about 300° C. for 30 minutes.

The foils are then edge formed again in an aqueous phosphateelectrolyte, preferably ammonium dihydrogen phosphate (2^(nd) edgeformation). The concentration of the phosphate in water is from about0.01 wt % to about 5 wt %, preferably about 0.05 wt % to about 2 wt %,more preferably about 0.1 wt %. The temperature of the phosphateelectrolyte is from about 0° C. to about 90° C., preferably about 25° C.to about 90° C., more preferably about 55° C. The time of formationdepends on the temperature and concentration and is typically from about3 minutes to about 20 minutes, preferably about 7 minutes. Thephosphates can be soluble phosphate salts of alkali metal, amine, orammonium cations. Preferably, the electrolyte is ammonium dihydrogenphosphate at a concentration of from about 0.01 wt % to about 5 wt %.Optionally, the phosphate electrolyte can contain glycerine to preventany airline corrosion of the foil (Melody et al., US S/N).

After the formation in phosphate, the foils are given a final rinse inwater and dried to remove excess water.

EXAMPLE 1

Etched foil with a formed layer such that the withstanding voltage was13 V (capacitance ˜119 μF/ cm²) was slit to a width of 3 mm. The foilwas cut to a length of 11 mm and attached to stainless steel carrierbars. A polyimide masking material was applied to each of the foilelements on the carrier bar so that an area of 3 mm×6.1 mm was definedon each foil element.

The carrier bars were divided into four groups. Each group was edgeformed in the electrolytes shown in Table I. Group 1 was edge formedaccording to the process flow of FIG. 2. Groups 2, 3, and 4 were edgeformed according to the process flow of FIG. 1. Each group was hydratedin deionized water for 90 minutes at 70° C. The foils were then reformedin 9% ammonium adipate (at 50° C.) for 24 minutes and the charge underthe reformation curve was calculated from the measured current. The lastcolumn of Table I shows the calculated charge in millicoulombs persquare cm of geometric surface area.

Group 1, anodized in oxalic acid followed edge formation in ammoniumadipate, was severely discolored and had a large capacitance decrease(capacitance went from 17.7 to 3.2 μF/element) after the hydration test.A charge of >700 mC/cm² was passed during the reform after hydration.The color change is indicative of hydrated oxide formation. The largecapacitance decrease occurs because of the formation of massive amountsof hydrated oxide, which plug the fine pores of the etched foil.

In contrast, Groups 2, 3, and 4, that had no oxalic acid anodization andwere edge formed in ammonium citrate or ammonium dihydrogen phosphate,were not discolored, had little change in capacitance, and the chargepassed during the reformation was ˜50 to 100 times less than the case ofoxalic acid/ammonium adipate formation.

TABLE I 1^(st) Edge 2^(nd) Edge Reform Charge After AnodizationFormation Formation Hydration mC/cm² OA AA AA 701 None AC AC 23.2 NoneAC ADP 10.7 None ADP ADP 5.87 OA = 5% oxalic acid (Room temperature) AA= 9% ammonium adipate (50° C.) AC = 1% dibasic ammonium citrate (55° C.)ADP = 0.1% ammonium dihydrogen phosphate (55° C.)

EXAMPLE 2

Three batches of multi-layer aluminum capacitors with a conductivepolymer cathode were fabricated. Aluminum foil was etched, formed to awithstanding voltage of 13 volts, and slit to 3 mm in width. The foilwas then cut into 11 mm lengths and attached to carrier bars. A maskingline was applied to the foil. Each batch was then divided into twogroups. One group was edge formed in ammonium dihydrogen phosphate usingthe process flow in FIG. 1. The other group was anodized in the priorart electrolyte system of oxalic acid followed by edge formation inammonium adipate using the process flow in FIG. 2.

A second masking line was applied. A conductive polymer layer of poly(3,4-ethylenedioxythiophene) was applied by chemical polymerizationusing techniques known to those skilled in the art (U.S. Pat. No.4,910,645, Jonas et al.). The capacitors were then rinsed ofpolymerization byproducts and carbon and silver paste layers wereapplied. The capacitor elements were cut off the carrier bar. Thecathode end of the capacitors were attached to the lead frame with asilver adhesive and the positive ends were welded to the lead frame byconventional resistance welding techniques. Four capacitors wereattached to each lead frame to make a 4-layer device. The capacitorswere then encapsulated in an epoxy case by transfer molding.

Table II shows the capacitance of the devices after molding. Thecapacitance of the hydration resistant formation system of ADP was 9%less than the prior art system using oxalic acid anodization followed byedge formation in ammonium adipate. This is disadvantageous as highcapacitance in a given package volume is desired.

TABLE II 1^(st) Edge 2^(nd) Edge Capacitance Anodization FormationFormation (μF) None ADP ADP 48.15 OA AA AA 52.85

EXAMPLE 3

Two batches of capacitors were fabricated in a similar manner to Example2. One half of each batch was edge formed in AC electrolyte using theprocess flow in FIG. 1. The other half of each batch was anodized in theprior art electrolyte system of OA followed by edge formation in AAusing the process flow in FIG. 2. The average capacitance of the twobatches is shown in Table III. In this case, the capacitance was only3.6% less than for the OA and AA system.

TABLE III 1^(st) Edge 2^(nd) Edge Capacitance Anodization FormationFormation (μF) None AC AC 52.34 OA AA AA 54.32

EXAMPLE 4

Five batches of capacitors were fabricated in a similar manner toExample 2. One half of each batch was edge formed in AC electrolytefollowed by ADP electrolyte using the process in FIG. 1. The other halfof each batch was anodized in the prior art electrolyte system of OAfollowed by edge formation in AA using the process flow in FIG. 2. Theaverage capacitance of the five batches is shown in Table IV. Thecapacitance for the AC/ADP edge formation system was 4% less than forthe OA/AA system. This is similar to the capacitance difference inExample 3, but, as shown in Example 1, the hydration resistance of theAC/ADP system is better than the AC/AC system.

The capacitors were further tested by exposing them to a temperature of85° C. and a relative humidity of 85% for 168 hours. After exposure, theleakage current of the group processed in ammonium citrate and ammoniumdihydrogen phosphate was less than half that of the group processed inthe prior art system of oxalic acid and ammonium adipate.

TABLE IV Leakage Current 1^(st) Edge 2^(nd) Edge Capacitance After 168hrs. 85 Anodization Formation Formation (μF) C/85% RH (μA) None AC ADP51.48  5.0 OA AA AA 53.65 12.0

Thus, the edge formation electrolyte system of ammonium citrate followedby ammonium dihydrogen phosphate gives the best combination ofcapacitance and hydration resistance.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques that fallwithin the spirit and scope of the invention as set forth in theappended claims.

We claim:
 1. A solid electrolytic capacitor comprising a foil coatedwith a dielectric oxide film, wherein the coated foil has slit or cutedges, and the slit or cut edges have been reformed by forming the foilin an aqueous citrate electrolyte, then depolarizing the foil, and thenforming the foil in an aqueous phosphate electrolyte wherein the foil isnot anodized in an aqueous acid electrolyte prior to forming the foil inan aqueous citrate electrolyte.
 2. The capacitor of claim 1 wherein thefoil is aluminum.
 3. The capacitor of claim 1 wherein, prior to formingthe foil in the aqueous citrate electrolyte, the foils are heat-treated.4. The capacitor of claim 3 wherein the foils are heat-treated at atemperature of from about 250° C. to about 550° C. for about 2 minutesto about 12 hours.
 5. The capacitor of claim 4 wherein the foils areheat-treated at a temperature from about 300° C. to about 350° C. forabout 15 to about 30 minutes.
 6. The capacitor of claim 1 wherein thecitrate is ammonium citrate.
 7. The capacitor of claim 1 wherein theconcentration of citrate in the aqueous citrate electrolyte is fromabout 0.1 to about 10%.
 8. The capacitor of claim 1 wherein thetemperature of the aqueous citrate electrolyte is from about 0 C. toabout 90° C.
 9. The capacitor of claim 1 wherein the concentration ofphosphate in the aqueous phosphate electrolyte is from about 0.1 toabout 10%.
 10. The capacitor of claim 1 wherein the temperature of theaqueous phosphate electrolyte is from about 0 C. to about 90° C.
 11. Thecapacitor of claim 1 wherein the phosphate is ammonium dihydrogenphosphate.
 12. The capacitor of claim 11 wherein the concentration ofthe ammonium dihydrogen phosphate in the aqueous phosphate electrolyteis about 0.01% to about 5%.
 13. The capacitor of claim 1 wherein theaqueous phosphate electrolyte further comprises glycerine in an amountto prevent airline corrosion of the foil.